Various new iron and other metal-making processes have been proposed to replace conventional smelting and blast furnace reduction processes. These new iron and other metal-making processes involve pre-reducing metal oxide agglomerates with a carbonaceous material in a rotary hearth furnace to form reduced agglomerates, and then melting the reduced agglomerates in an arc furnace or submerged arc furnace. See, for example, WO/2000/513411, WO/2001/525138, WO/2001/525487, and WO/2003/105414.
However, in the processes that use an arc furnace as the melting furnace, metallization of the reduced agglomerates must be maintained at a high level and the fines ratio must be maintained at a low level to ensure high melting efficiency, refractory protection, excessive foamy slag formation suppression, and the like. Thus, using these processes, it has been difficult to increase the productivity of the rotary hearth furnace while maintaining higher metallization levels and lower fines ratio levels. Further, the facility associated with these processes is necessarily large.
By contrast, in the processes that use a submerged arc furnace as the melting furnace, the reduced agglomerates form layers, and damage to the refractory and excessive foamy slag formation are less problematic. Further, there are lesser limitations on the metallization and fines ratio levels, and the facility associated with these processes may be considerably smaller. However, in the processes that use a submerged arc furnace as the melting furnace, it is difficult to effectively use the chemical energy of the CO gas generated by the reduction step remaining in the reduced agglomerates. Thus, productivity cannot be sufficiently increased and operation cost cannot be sufficiently decreased.
In the processes that use a submerged arc furnace as the melting furnace, it is possible to omit the rotary hearth furnace pre-reduction step and charge the un-reduced metal oxide agglomerates, with the carbonaceous material, directly to the submerged arc furnace, such that the pre-reduction step and the melting step are performed in the same furnace. However, when the metal oxide agglomerates and carbonaceous material contain volatile metal elements in addition to nonvolatile metal elements that form the molten metal (i.e. when iron mill dust or the like is used as the metal oxide raw material), the volatile metal elements evaporated and removed from the reduced agglomerates in the lower region of the furnace re-condense in a low-temperature zone in the upper region of the furnace and circulate in the furnace by adhering to the reduced agglomerates and/or forming accretions on the walls of the furnace. Thus, it is possible that the volatile metal elements cannot be efficiently recovered from the exhaust gas. In addition, non-descending reduced agglomerates may cause operational problems.
Accordingly, in these processes, two steps are typically used (a pre-reduction step using a rotary hearth furnace and a melting step using an arc furnace or submerged arc furnace). These processes require facilities and equipment for transferring the reduced agglomerates from the rotary hearth furnace to the melting furnace, as well as two exhaust gas processing lines, i.e. one for the rotary hearth furnace and one for the melting furnace. Thus, facility and equipment cost is high, thermal loss is high, and total system energy consumption cannot be adequately minimized.
As a result, a method for producing molten metal using a stationary non-tilting electric furnace has been proposed in JP/2009/280910. This method involves using a raw material charging chute that is provided at one end of the furnace, width-wise, that is connected to the interior portion of the furnace through its upper portion, an electric heater that heats the lower portion of the furnace and is located opposite the raw material charging chute, width-wise, and a secondary combustion burner disposed at the upper portion of the furnace between the two ends, width-wise. The method includes forming a raw material layer by charging a predetermined amount of a carbonaceous material and/or metal oxide agglomerates with carbonaceous material containing a nonvolatile metal element that forms molten metal into the furnace using the raw material charging chute, with a downward sloping surface extending from one end of the furnace to the other, subsequently forming an agglomerate layer on the downward sloping surface by charging a predetermined amount of the metal oxide agglomerates with carbonaceous material into the furnace using the raw material charging chute, subsequently forming a molten metal layer and a molten slag layer in the furnace by heating the lower end of the agglomerate layer with the electric heater while allowing the agglomerate layer to descend along the downward sloping surface toward the lower end of the furnace by melting, and concurrently thermally reducing the agglomerate layer by radiant heat from secondary combustion by blowing oxygen-containing gas into the furnace to burn the CO-containing gas generated by the agglomerate layer.
This stationary non-tilting electric furnace is illustrated in FIG. 1. The furnace 10 is an arc furnace that has a substantially-rectangular cross-sectional shape, for example. Raw material charging chutes 12 and exhaust gas ducts 14 are connected to/through the top wall 16 of the furnace 10. Electrodes 18 that function as heaters are inserted through the top wall 16 of the furnace 10. The raw material charging chutes 12 are provided adjacent to both side walls 20 of the furnace 10, for example, with the electrodes 18 provided near the centerline of the furnace 10. Multiple raw material charging chutes 12 and electrodes 18 may be spaced along the length of the furnace 10. Secondary combustion burners 22 are also inserted through the top wall 16 of the furnace 10. Multiple exhaust gas ducts 14 and secondary combustion burners 22 may be spaced along the length of the furnace 10. Preferably, the exhaust gas ducts 14 are disposed closer to the raw material charging chutes 12 than the electrodes 18 in order to prevent oxidizing exhaust gas produced after secondary combustion from flowing towards the electrodes 18, thereby mitigating damage to the electrodes 18.
Referring to FIG. 2, in the side walls 20/bottom wall 24 of the furnace 10, near the centerline and distant from the raw material charging chutes 12 (i.e. distant from the raw material beds 30 (FIG. 1)), a metal tap hole 26 and a slag tap hole 28 are provided to facilitate the tapping of molten metal 32 (FIG. 1) and molten slag 34 (FIG. 1). The electrodes 18 are preferably of a three-phase alternating-current type that has desirable heat efficiency, as is typically used in steel-making electric arc furnaces. As an example, an array of six electrodes 18 may be used, consisting of three pairs of electrodes 18 each of a single phase. The tip portion 36 (FIG. 1) of each electrode 18 is preferably submerged in the agglomerate layers 38 (FIG. 1) disposed on the raw material beds 30, or submerged in the molten slag 34, while conducting the melting operation. As a result, melting can be accelerated by the effects of radiant heat and resistance heat, and damage to the interior surfaces of the furnace 10 that are not protected by raw material beds 30 can be minimized.
Referring to FIG. 3, in operation, it is necessary to control the material flow and the position of the melting area in the furnace 10. Thus, the raw material charging chutes 12 are equipped with outer chutes 40 including feeding ports 42 that may be telescoped or otherwise adjusted vertically. Each raw material charging chute 12 includes a hopper 44 for storing the raw material, an inner chute 43 connected to the hopper 44, and an outer chute 40 that can be telescoped or otherwise adjusted vertically on the inner chute 43. The lower portion of the agglomerate layer 38 may be adjusted to occur at a desired position by moving the outer chutes 40 and feeding ports 42 in a vertical direction, depending on the angle of repose of the agglomerate layer 38.
One problem that can occur is that fines can accumulate in the furnace 10. Fines enter the furnace 10 with the feed material and/or are generated in the furnace 10 due to movement of the layer, thermal stresses, etc. These fines are segregated in the furnace 10 and increase the angle of repose of the agglomerate layer 38. If the fines are not removed, then continuous operation of the furnace 10 over long periods of time cannot be maintained due to changes in the angle of repose and unstable material flow. Feed leg height adjustment, as described above, cannot be used to satisfactorily position and control the agglomerate layer 38 if excess fines have accumulated in the furnace 10. Thus, additional techniques to deal with fines have been proposed, such as: shocking the agglomerate layer 38 using a hammer or the like, utilizing air blasts, or utilizing other means to correct and control material flow disruptions. Again, continuous operation is necessarily limited if fines are not periodically dealt with and/or removed. In order to remove fines, operation of the furnace 10 must be halted and the top wall 16 (FIG. 1) or side walls 20 of the furnace 10 must be opened. This operation is difficult, as the furnace 10 is hot and the material may readily oxidize.
Thus, what is still needed in the art is an electric furnace for producing molten metal that has material recycling capability, especially in-process material recycling capability, such that fines or other material may be removed from the furnace periodically, without shutting it down for extended periods to allow for cooling, etc.